xC: make alias creation tolerant to prefixed names

Those would not work, so skip the first forward slash.

Note that liberty can save arbitrary alias names since 6e93119,
making the removed comment about checking outdated.

xC.c

@@ -11547,10 +11547,11 @@ handle_command_alias (struct handler_args *a)
 	if (!*a->arguments)
 		return show_aliases_list (a->ctx);
 
-	// TODO: validate the name; maybe also while loading configuration
 	char *name = cut_word (&a->arguments);
 	if (!*a->arguments)
 		return false;
+	if (*name == '/')
+		name++;
 
 	struct config_item *alias = config_item_string_from_cstr (a->arguments);